Decide on the features you require first, which will tell you if you need Enterprise or Standard. You should be able to get this list from your third-party vendor if you are deploying an off-the-shelf product. Typically Standard is fine unless otherwise stated, but always make sure.
Second you need to decide what size box you will deploy on and what architecture. If you are looking at per-processor licensing, know that a typical Intel i386 or i386-x64 the core scale is .5, so if the processor has 4 cores, you actually have to cover it with 2 processor licenses. This will also determine if you can get away with using Standard Edition One, which has a max of 2 cpu sockets support (no other limits) but is considerably cheaper than Standard Edition. Effective Core Factor Table
Third, you need to decide if you can get away with Named User licensing. You say that you are looking for a solution for 200 people, so you have some use in mind already. If those are it, named user plus pricing may be for you. Cost wise, if you are able to do Standard Edition One, then named user plus doesn't pan out even based on that, as the current retail price of named users on Standard Edition One is 180 per person ($180.00 x 200 = $36,000). On Intel i386/x64 architecture, with 4 core processors, your cost would be $23,200 ($5,800 per processor x .5 core factor x 8 cores). Additional information on Named User vs Per Processor
If you are going to have public internet based applications, then you are definitely into the per-processor licensing, since you cannot identify the different people that may connect. Private internet applications would be a different story.
Lastly, you need to decide if you require many of the features that you may add on to your Oracle Database purchase. Before you dive into this hole, you may want to look at third-party applications that may offer some of the same features as well. Things like data guard are great, and offered from Oracle on Enterprise edition only, but there are companies that offer a product that will do very similar things with Standard Edition, which is a significant cost reduction if all you wanted was Active/Passive fail over.